What percentage of air is composed of oxygen?

Oxygen constitutes approximately 21% of the Earth's atmosphere, making this the correct choice. This percentage is significant because oxygen is vital for the survival of most living organisms, as it is necessary for processes like respiration in animals and photosynthesis in plants. Understanding the composition of air is crucial for agriculture and environmental science, as it directly influences plant growth and metabolism.

The other percentages provided do not accurately reflect the actual composition of Earth’s atmosphere. For instance, 20% and 25% are slightly off, and the 15% figure is considerably lower than the established measurement. Knowing the precise percentage of oxygen helps in various fields such as ecology, respiratory physiology, and agriculture.